Skyscrapers peek through construc tion at the Transbay Transit Center.

Skyscrapers peek through construc tion at the Transbay Transit Center.

Photo: Michael Macor / The Chronicle

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Height limits within the 145-acre Transit Center District south of Market Street would be raised to create a new southern skyline of office and residential towers.

Height limits within the 145-acre Transit Center District south of Market Street would be raised to create a new southern skyline of office and residential towers.

Photo: Michael Macor, The Chronicle

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Nearly three blocks of construction continues on the Transbay Transit Center South of Market St. in San Francisco, Ca., on Thursday May 17, 2012.

Nearly three blocks of construction continues on the Transbay Transit Center South of Market St. in San Francisco, Ca., on Thursday May 17, 2012.

Photo: Michael Macor, The Chronicle

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The proposed Transbay Tower at First and Mission streets calls for a 1,070 foot high building -- more than 200 feet taller than the Transamerica Pyramid, now San Francisco's tallest building. The top 150 feet are hollow. less

The proposed Transbay Tower at First and Mission streets calls for a 1,070 foot high building -- more than 200 feet taller than the Transamerica Pyramid, now San Francisco's tallest building. The top 150 feet ... more

Photo: Pelli Clarke Pelli

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Workers already have plenty to do, but details of the Transit Center District development are still being worked out. Tentative plans include a 1,070-foot skyscraper at First and Mission streets, with a half-dozen towers around it. less

Workers already have plenty to do, but details of the Transit Center District development are still being worked out. Tentative plans include a 1,070-foot skyscraper at First and Mission streets, with a ... more

Photo: Michael Macor, The Chronicle

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Workers busy on the scene as construction continues on the Transbay Transit Center South of Market St. in San Francisco, Ca., on Thursday May 17, 2012.

Workers busy on the scene as construction continues on the Transbay Transit Center South of Market St. in San Francisco, Ca., on Thursday May 17, 2012.

Photo: Michael Macor, The Chronicle

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Nearly three blocks of construction continues on the Transbay Transit Center South of Market St. in San Francisco, Ca., on Thursday May 17, 2012.

Nearly three blocks of construction continues on the Transbay Transit Center South of Market St. in San Francisco, Ca., on Thursday May 17, 2012.

Photo: Michael Macor, The Chronicle

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Remnants of the past as construction continues on the Transbay Transit Center South of Market St. in San Francisco, Ca., on Thursday May 17, 2012.

Remnants of the past as construction continues on the Transbay Transit Center South of Market St. in San Francisco, Ca., on Thursday May 17, 2012.

Photo: Michael Macor, The Chronicle

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Transbay plan - is new skyline worth more shadows?

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San Francisco has never seen a neighborhood plan as ambitious as the one that goes to the Planning Commission on Thursday, or one that so audaciously seeks to fit 21st century urban values into a 20th century frame.

Height limits within the 145-acre Transit Center District would be raised to create a new southern skyline of office and residential towers amid new public spaces and transit services of every sort. There'd be widened sidewalks, new pedestrian crossings and bicycle lanes, perhaps even a district-wide geothermal energy plant.

That's why it is critical that the first building proposed under the plan's framework, a tower that would be the city's tallest, embody the values of an area intended by the city to be "an enjoyable and humane place to spend time."

Planners also need to be candid about the disruptive aspects of the plan that, whatever its other merits, would clear the way for towers casting new shadows across parks as far as Chinatown.

Those far-reaching shadows are the most troubling aspect of a proposal that in many ways deserves to be embraced.

The planning effort that began in 2007 in many ways is a response to what now is a 20-foot-deep hole in the ground: a transit terminal set to open in 2017 off Mission Street that would be cloaked in futuristic curves of glass. Buses from throughout the Bay Area will pull in on opening day. A second phase would include underground train tracks to make the station the final stop for commuter trains from the Peninsula and high-speed rail from Southern California.

Heart of the city

The goal of the plan is to build on the convenient transit access and make the area "the geographic heart and center" of downtown San Francisco. The increased heights would allow for a half-dozen towers of 700 to 850 feet clustered around a 1,070-foot skyscraper at First and Mission streets. That's 217 feet taller than the Transamerica Pyramid, the downtown skyline's summit since 1972.

"The tower represents the city's commitment to focusing growth around a sustainable transportation hub," reads the draft of the plan released in 2009 and scheduled for a vote on Thursday. The Board of Supervisors also must approve certain legal aspects of the plan, such as zoning changes and the fees that developers would be charged for improvements in and near the district.

Economics are as important as iconography: According to city estimates, development fees and land sale proceeds from the new zoning would generate more than $600 million for the terminal and its rail connections, a project with a daunting $4 billion price tag.

In the shadows

The catch? Buildings of this scale would cast early morning shadows that could cover portions of St. Mary's and Portsmouth squares in Chinatown for up to an hour during portions of the late fall and early winter.

The impact on Portsmouth Square - which the environmental impact report says new shadows would be "readily noticeable to park users" - is particularly significant because the busy plaza at Kearny and Clay streets is a popular spot for tai chi at the start of the day.

Indeed, the desire to keep new shadows out of downtown parks spawned a 1984 ballot measure that requires explicit approval from the Planning and Recreation and Park commissions before any new shadows are cast on nine downtown parks.

Planners make the case that shadows from a distant building - the 1,070-foot tower would be one-half mile from Portsmouth Square - are diffuse rather than dark. It's a distinction that should be explained and illustrated when facets of the plan go to the board this summer for a final round of hearings.

On balance, the planners make a strong case that the overall benefits outweigh the impact of additional shadows part of the day, part of the year.

Adding new parks

For starters, there's the addition of park space to an area that now has none. The centerpiece would be the transit terminal's 5.4-acre rooftop park, billed as "a self-sustaining ecosystem." Four other small parks would be added at ground level, including Mission Square, a half-acre at Fremont and Mission with the so-called Transbay Tower on the west edge and the terminal's main entrance forming the south side.

The plan also would steer development fees to improvements outside the district's boundaries. There is $10 million allocated to ease the pedestrian congestion at BART's Embarcadero and Montgomery stations - and $12.5 million for improvements to downtown parks, with Portsmouth and St. Mary's squares at the top of the list.

Making good neighbors

While shadows are important, another challenge of the plan, if approved, is to stay vigilant. New buildings must be good neighbors as well as skyline shows.

This issue already arose in connection with the Transbay Tower, which could go to the Planning Commission for its own approvals this fall.

When developer Hines proposed the tower in 2007, the pitch included cable car-like people movers from Mission Square to the rooftop park. That user-friendly link was missing when tower plans were submitted to the city in March.

Planning commissioners made it clear this month that the rooftop park can't be an isolated Eden unto itself - and Hines got the message. The people-movers have been restored.

This type of tension is likely to recur. Planners, rightly, see an obligation for developers to create a richer public realm in return for added heights. Those developers, by contrast, will be watching out for their own projects.

What needs to be remembered is that the terminal and these towers are being allowed because the city has agreed to complicated and sometimes painful trade-offs. The public must not be treated as second-class citizens.

Planning hearing

The San Francisco Planning Commission hearing on the Transit Center District Plan is at 10 a.m. Thursday, Room 400 at City Hall. For more information on the plan and related studies, go to bit.ly/o6ud1D.